In this country schizophrenia has been consistently overdiagnosed and affective disorders underdiagnosed, particularly among blacks and lower socioeconomic groups. The general causes of such misdiagnoses include overreliance on the classic thought disorder symptoms as pathognomonic of schizophrenia and, for affective disorders, lack of clearly defined boundaries between normal and abnormal mood and failure to realize that patients with affective illness can manifest cognitive symptoms. In addition to the above factors, misdiagnosis among blacks results from such factors as cultural differences in language and mannerisms, difficulties in relating between black patients and white therapists, and the myth that blacks rarely suffer from affective disorders. Clinicians and researchers must pay more attention to the effects of cultural differences on diagnosis, and baseline behaviors and symptomatology for blacks must be established.